does anyone know how liquid the full size electronic silver contract from the CBOT is ? Is it liquid enough to trade stops with it (without getting slaughtered) ?

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It does a few thousand contracts per day. The bid-asked spread is not bad but not great either. I'd say it is liquid enough for limit orders or stop-limit orders and maybe small stop orders during the liquid part of the day.

interesting point about the smaller size electronic contracts - but I would think the full size electronic contracts would have more volume than the smaller ones - but I am just guessing - does anyone know ?

It seems silver spreads, at least in the mini contracts, have gotten wider since last year, at least in terms of where the arbs are straddling (30-40 ticks as opposed to 20-30). But I suppose that is natural for the increased volatility. The volume for the large contract was pitiful in the months after they began -- maybe setharb knows if ZI is actually tradable now?

I trade them both, personally I'd be very careful about using stops in the evening and overnight. The bid/ask is usually decent in the front month, but there have been a few days where I have seen this market move much more than its counterpart at the comex.